Gynaecomastia is male breast enlargement as a result of hormonal changes during puberty.
Research among 536 boys between 10-15 years of age showed 48,5% to have swollen breasts. So this is perfectly normal, although it may be embarrassing.
It normally starts as a tender lump under the nipple. Later on the area around the nipple may swell.
In more than half the cases both breasts are enlarged, but it can also be unilateral.
The enlargement normally disappears spontaneously in a few months to two years. If your breasts remain enlarged you should visit your doctor to find out if anything is wrong.
Breast enlargement outside puberty requires appropriate assessment.
Cannabis, heroin and anabolic steroids may also cause breast enlargement.

I am 14 and have a lump under my right nipple. Can it be breast cancer? I am scared!
I’m a man of 26 and I have large breasts, almost like a woman. Can I do anything about it?